Obviously Naruto would be one of the main characters. He would bring his ninja skills and his unique personality to the Monster High setting. Then, characters like Frankie Stein could be important. She is curious and open - minded, so she would be likely to befriend Naruto quickly and might be part of the harem. Also, Cleo de Nile, with her strong personality and leadership qualities, could play a major role. She might be initially skeptical of Naruto but then grow to respect and like him, becoming another member of the harem.
In addition to Naruto, Ghoulia Yelps could be a key character. Her intelligence would be useful in helping Naruto understand the Monster High world better, and she might be drawn to his heroic nature. Spectra Vondergeist could also be part of the story. Her ability to move through walls and be a bit of an outcast could make her bond with Naruto, who also has felt like an outsider at times. And of course, the Monster High headmistress Bloodgood could have a role in overseeing Naruto's stay and the development of the relationships that form the harem.
Tenten could be a main female character. She could be the sporty girl in high school who challenges Naruto to various athletic competitions and during that time, starts to develop feelings for him. Ino could also be part of the harem, as the fashionable and sassy girl who tries to win Naruto over with her charm and style.
One potential monster girl could be a centaur - like being. With the lower body of a horse and the upper body of a human - like monster. She would be strong and have great stamina for long travels. And then there could be a phoenix - like monster girl. She could have the power of fire and rebirth. Whenever Naruto is in danger, she can use her fire powers to protect him and then rejuvenate herself if she gets injured.
Naruto and Ciri could be key characters. Naruto's ability to make friends easily and his never - give - up attitude would be appealing to Ciri. Ciri, with her unique powers and destiny, would be an interesting addition to the Naruto world. For the harem aspect, Sakura from Naruto could be one of those in Naruto's harem, while Triss from the Witcher could be part of Ciri's (if we consider a more complex relationship dynamic) harem. And they could all be involved in a story about protecting their combined worlds from a great evil.
From Naruto, Tenten could be in the harem. She is a skilled weapon user. In the DBZ universe, Marron could be part of it. She is Krillin's daughter and could have her own story arc in this harem situation. Also, Pan from DBZ, being Goku's granddaughter, could be an interesting character in the harem with her Saiyan blood and adventurous spirit.
He would surely interact with Lagoona Blue. Since Naruto loves water-based techniques, he might be interested in Lagoona's mermaid - like abilities. Also, he could meet Cleo de Nile. Naruto would probably be amazed by her confidence and the fact that she's a mummy. They could have some really interesting conversations about their different cultures and powers.
There could be a kitsune - like monster girl, with her fox - like features and tricky nature. Another might be a lamia, half - snake half - human, who could have unique abilities related to snakes, like venom or constriction. And perhaps a mermaid - type monster girl, who could bring in the element of water and her own underwater world lore.
Well, you could have a fox - girl. Given Naruto's connection with the nine - tailed fox, it would be interesting to have a fox - girl in the harem. Maybe she has some of the same powers as the nine - tailed fox but in a more feminine and alluring way.
Well, a lamia - a half - snake half - human monster girl - would be a great addition. She could use her snake - like abilities to help Naruto in stealth or tracking missions. There might also be a kitsune - a multi - tailed fox - girl. Given Naruto's relation to the nine - tailed fox, there could be an interesting connection between them. And a manticore - girl, with her dangerous yet alluring nature, would also be a potential character.
Sasuke could be one of the main characters. Given his power and brooding nature, he could be drawn into the world of human puppets and end up in a harem situation. Maybe a puppet master sees his potential and tries to recruit him, and along the way, a group of female characters start to form a harem around him. Another possibility is Naruto himself. His kind heart and strong will could attract a harem while he's exploring the new concept of human puppets. He might try to use the puppets for good, and in the process, gather a group of followers who become part of his harem.
Obviously Naruto would be one of the main characters as it's his world and he's the center of the harem. Hinata would likely be a part of it too, given her strong feelings for Naruto in the original series. Sakura could also be included, as she has a long - standing relationship with Naruto as well.